It depends on the number of your guests. Sit down meals are a little more elegant. But if you have more than 50 guests, it will take so much time to serve them one by one.
I have to agree with Jessie that sit down meals are more elegant then buffet. The meal is arranged with style on the plate, and your guests don't need to fall in line for their food. But when it comes to big weddings, the sit down style can be impractical.